Silicon germanium hetero bipolar transistor having a germanium concentration profile in the base layer

ABSTRACT

The invention relates to a silicon germanium hetero bipolar transistor and a method of fabricating the epitaxial individual layers of a silicon germanium hetero bipolar transistor.  
     Silicon germanium hetero bipolar transistors thus fabricated have an increased transit frequency, an increase maximum oscillation frequency and/or a reduced noise level depending upon requirements and intended application.  
     A monocrystalline deposition is performed on a surface of pure silicon in accordance with the desired transistor profile. The silicon germanium hetero bipolar transistor contains an additional electrically inert material The semiconductor arrangement of silicon germanium hetero bbipolar transistors is fabricated by an epitaxy process. A electrically inert material incorporated into the epitaxial layer links fabrication defects and reduces the diffusion of the dopant. Thus, transistors for high frequency applications may be fabricated in two ways: The dopant dose of the base region is increased and/or the width of the base is reduced.

[0001] The invention relates to a silicon germanium hetero bipolar transistor for high frequency applications and to a method of fabricating the epitaxial individual layers of a silicon germanium hetero bipolar transistor for high frequency applications.

[0002] Aside from using gallium arsenide for fabricating super high frequency transistors, silicon germanium hetero bipolar transistors, because of their lower fabrication costs, have found increased use in high frequency areas. The sequence of layers in such transistors generally consists of a silicon collector layer, a base layer of p-doped silicon germanium, and an emitter layer.

[0003] German laid-open patent specification 43 01 333 A1 describes a method of fabricating integrated silicon germanium hetero bipolar transistors in which a collector layer, a base layer, an emitter layer and an emitter connection layer are precipitated and doped at the same time in a single uninterrupted process. This method of fabricating transistors for high frequency applications suffers from the drawback that a further increase in the doping of the base with doping atoms would lead to an outdiffusion, i.e. a broadening of the base region, at a corresponding temperature. Outdiffusion of dopants, on the one hand, results in a non-uniform fabrication of transistors and, on the other hand, in a reduction of collector and emitter currents. Accordingly, it is not possible by this method to improve the high frequency properties of transistors. Also, broadening of the doped regions limits a further reduction of the structure.

[0004] Japanese patent application JP 5,102,177 discloses a silicon germanium hetero bipolar transistor the base of which has been dislocated by 5% of carbon to compensate for mechanical strain introduced by the germanium. However, such high carbon concentrations result in a strong local lattice deformation which limits the suitability of such transistors for high frequency applications.

[0005] Also, in IEEE Electron Device Letters, Vol. 17, No. 7, July 1996, pp. 334-337 as well Appl. Phys. Lett., vol. 60, No. 24, pp. 3033-3035, 1992 and Meter. Lett., Vol. 18, pp. 57-60, 1993, carbon is incorporated into the base for the purpose of attaining current compensation of germanium in silicon by carbon as well as a variation in the band gap. Optimum results were found at a carbon concentration of 5-10²⁰ cm⁻³. Drawback similar to those of the above-mentioned JP 5,102,177 may be expected. In IEEE Electron Device Letters, Vol. 17, No. 7, July 1996, pp. 334-337, large surface MESA transistors with emitter surfaces of ≧400 μm² (line width ≧20 μm) were used to define static component properties. Such transistors with large emitter surfaces do not satisfy high frequency applications.

[0006] To fabricate SiGe transistors suitable for high frequency applications line widths less than 2 μm are necessary as disclosed, for instance, in T. F. Meister: SiGe Base Bipolar Technology with 74 Ghz f_(max) and 11 ps Gate Delay; IEDM95-739.

[0007] U.S. Pat. No. 5,378,901 discloses a silicon carbide transistor in which silicon carbide is used as the material for the base, collector and emitter. The high fabrication temperatures prevent their integration into circuits suitable for high frequency applications.

[0008] It is a task of the invention to provide a silicon germanium hetero bipolar transistor suitable for high frequency applications in which the outdiffusion of dopant from the base region is reduced by more than 50% compared to conventional silicon germanium hetero bipolar transistors. It is a further task of the invention to structure known methods of fabricating the epitaxial individual layers of such silicon germanium hetero bipolar transistor suitable for high frequency applications with a silicon collector layer, a doped silicon germanium base layer and a silicon emitter layer so as to reduce the usual limitations and complex requirements of subsequent processes. This refers especially to the implantation dose and the temperature time stress of the epitaxial layer. Silicon germanium hetero bipolar transistors made by this method have a higher transitory frequency, an increased maximum oscillation frequency and/or a reduced noise level depending upon requirements and intended application.

[0009] Furthermore, it is a task of the invention by a point defect supported diffusion acceleration to prevent boron outdiffision from the silicon germanium layer, in order to attain HF properties without losses in a scaling range of a line width of 0.4 μm or less. In this manner, similar transitory and maximum oscillation frequencies are to be attained compared to larger emitter surfaces.

[0010] In accordance with the invention these tasks are accomplished by the invention described hereinafter.

[0011] A monocrystalline structure according to a desired transistor profile is precipitated on a surface of pure silicon. In at least one of its three individual layers, i.e. its emitter layer or its base layer or its collector layer, the silicon germanium hetero bipolar transistor in accordance with the invention contains an additional material which is electrically inert, preferably an element from group IV, in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³. The semiconductor arrangement of the silicon germanium hetero bipolar transistor is fabricated by an epitaxy process, e.g. by vapor phase epitaxy or molecular-beam epitaxy. The technological process steps following the epitaxy lead to defects, e.g. interstitial atoms in the semiconductor crystal, which enhance the diffusion of atoms foreign to the lattice, such as dopants. An electrically inactive material of the kind referred to and incorporated into the epitaxial layer links these defects and reduces the diffusion of the dopant. The relative alteration of the lattice constant caused by the incorporation of an electrically inert material, preferably carbon, is less than 5-10⁻³. The outdiffusion of the dopant is reduced which limits broadening of the base region. This allows fabrication of transistors suitable for high frequency applications in two ways: The dopant dose of the base region is increased and/or the width of the base is reduced. Either way leads to an increase in the concentration of dopant in the base region of the transistor by between 5-10¹⁸ cm⁻³ and 10²¹ cm⁻³ if the dopant used is boron. This leads to a reduced resistance of the inner base. The invention proceeds on the basis of the conventional fabrication of a preprocessed silicon substrate. The method is characterized by the following process steps: Initially silicon is deposited by vapor deposition for fabricating the collector layer. Germanium is additionally incorporated during the following further silicon vapor deposition and is doped with lattice doping atoms. The preferred dopant used is boron. The base is fabricated by this process step. After discontinuing the addition of boron and the doping medium the emitter layer is fabricated by further vapor deposition of silicon.

[0012] During at least one of the mentioned process steps an electrically inert material, preferably carbon, is added in a concentration of between 10¹⁸ cm⁻³ and 10²¹ cm⁻³, the relative change in the lattice constant thus introduced being less than 5-10⁻³ owing to the low concentration of the electrically inert material. A low additional lattice distortion does not imply an additional source of possible lattice defects. CVD (chemical vapor deposition) and MBE (molecular-beam epitaxy) processes are used to fabricate the epitaxial layers. Conventional further processing is carried out terminating in the silicon germanium hetero bipolar transistor in accordance with the invention. In silicon germanium hetero bipolar transistors in accordance with the invention the product of the germanium concentration in the base layer and the width of the base layer from the collector to the emitter is between 50 atomic percent nm and 2,000 atomic percent nm. The width of the base layer from the collector to the emitter is between about 5 nm and 60 nm and, preferably, between 35 nm and 40 nm. The concentration of germanium in the base layer is between about 8% and 30% and, preferably, between 20% and 28%.

[0013] The elements of the invention have not only been set forth in the claims but also in the description and in the drawings, whereby individual elements constitute patentable inventions not only by themselves but also when combines as subcombinations, the protection of which is here being applied for. Embodiments of the invention have been depicted in the drawings and will be described in greater detail hereinafter. In the drawings:

[0014]FIG. 1 is a schematic rendition of the layer structure of a silicon germanium hetero bipolar transistor;

[0015]FIG. 2 depicts steps of the method of fabricating the epitaxial individual layers of a silicon germanium hetero bipolar transistor;

[0016]FIG. 3 is a schematic section through a silicon germanium hetero bipolar transistor;

[0017]FIGS. 4, 5, 6 depict concentration curves of germanium in silicon germanium hetero bipolar transistors.

[0018] In FIG. 1, there is shown the layer structure of a silicon germanium hetero bipolar transistor in accordance with the invention, consisting of a doped silicon substrate 1, a non-doped silicon carbon collector layer 2, a doped silicon germanium carbon layer 3 and a non-doped silicon carbon emitter layer 4. The entire layer structure including doping of the base region with boron is fabricated by molecular beam epitaxy.

[0019] In this embodiment, carbon in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ is added simultaneously with the epitaxy of all three individual layers, the collector layer, the base layer and the emitter layer. This corresponds to a carbon concentration of between 0.0015% and 1.5%. In this manner an otherwise possible diffusion of boron is significantly reduced so that the regions of dopant outdiffusion 5 may be reduced compared to conventional transistors of this kind. By the addition of carbon in accordance with the invention the diffusion length of boron is reduced by more than 50% compared to a diffusion length occurring where no carbon has been added. The result is a very stable boron profile. The thus reduced base width results in a reduced base transit time. This, in the context of the transistor in accordance with the invention, is the same as an increased transit frequency and an increased maximum oscillation frequency or a reduced noise level.

[0020] By increasing the boron concentration to between 5-10¹⁸ cm⁻³ and 10²¹ cm⁻³ in the base layer the suitability of the silicon germanium hetero bipolar transistor may be further improved.

[0021] For fabricating such a silicon germanium hetero bipolar transistor the following process steps shown in FIG. 2 are performed: Prior to the part of the method in accordance with the invention a pretreated silicon substrate is conventionally made in a process step A₀. This is followed by the steps of:

[0022] A vapor deposition of silicon to fabricate the collector layer;

[0023] B Vapor deposition of silicon and additional incorporation of germanium and dopant for fabricating the base layer; and

[0024] C discontinuing germanium and dopant and vapor deposition for fabricating the emitter layer

[0025] whereby carbon in a concentration of between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ is incorporated during at least one of the process steps with the relative change of the lattice constant thus introduced being less than 5-10 ⁻³.

[0026] The epitaxy is followed by convention further processing D terminating in a final silicon germanium hetero bipolar transistor in accordance with the invention.

[0027]FIG. 3 depicts a schematic section through a silicon germanium hetero bipolar transistor thus fabricated. The non-doped silicon carbon collector 32, the non-doped silicon carbon emitter 33 and the base 34 of silicon, germanium and carbon doped with boron in a concentration between 5-10¹⁸ cm⁻³ and 10²¹ cm⁻³ have been grown by epitaxy on the highly doped silicon substrate 31. Furthermore, the figure depicts the corresponding contact areas 35 as well as an implantation region 36. The carbon concentration in the epitaxial layer is between 10¹⁸ cm⁻³ and 10²¹ cm⁻³.

[0028] FIGS. 4 to 6 represent concentration curves of germanium in the silicon of silicon germanium hetero bipolar transistors in accordance with the invention. The curves are rectangular, triangular or trapezoidal. In all diagrams, the base region is limited on the abscissa by value x1 and x2. The ordinate represents the curve as a percentage of the germanium concentration.

[0029] In the transistor having the rectangular germanium concentration curve of FIG. 4 the width of the base layer is 30 nm. The concentration of germanium in the base layer is about 22% constant. Preferred high current amplifications and good dynamic properties may be obtained with this transistor profile.

[0030] In the transistor with the triangular germanium concentration curve of FIG. 5 the width of the base layer is 40 nm. The concentration of germanium in the center of the base layer where it reaches its maximum is about 26%. This transistor profile makes it possible to set very high early currents. Moreover, this transistor profile permits impressing a drift field for reducing the base transit time of the minority carrier.

[0031] In the transistor with the trapezoidal germanium concentration curve of FIG. 6 the width of the base layer is 35 nm. The concentration of germanium in the base layer increases linearly from the side of the collector or emitter of the transistor from about 10% to 22%. In this embodiment, high current amplification as well as high early current and a drift field are attained for reducing the transit time of the base.

[0032] At increased scaling broadening of the contact regions is avoided by the prevention of boron outdiffusion by carbon so that HF properties are preserved without losses in the scaling range of a line width of 0.4 μm or less. Compared to larger structures the same transit and maximum frequencies are attained here at lower currents.

[0033] In the context of the present invention a silicon germanium hetero bipolar transistor suitable for high frequency applications and a method of fabricating the epitaxial individual layers of such a transistor have been described with reference to concrete embodiments. It is, however, to be noted that the present invention is not restricted to the details of the description of the embodiments as changes and alterations are claimed within the metes and bounds of the patent claims. 

1. Silicon germanium hetero bipolar transistor suitable for high frequency applications, with a silicon collector layer, a doped silicon germanium base layer and a silicon emitter layer, characterized by the fact that carbon is incorporated in at least one of the three individual layers, i.e. the emitter layer and/or the base layer and/or the collector layer, in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ and that the relative change in the lattice constant thus introduced is less than 5-10⁻³ so that a point defect supported diffusion acceleration is prevented.
 2. Silicon germanium hetero bipolar transistor suitable for high frequency applications according to claim 1, characterized by the fact that the base layer is doped with boron and with a concentration of the dopant in the base region between 5-10¹⁹ cm⁻³ and 10⁻³ cm⁻³ the concentration of carbon in the epitaxy layer is between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ and the defect density of the transistor is less than 10⁴ cm⁻².
 3. Silicon germanium hetero bipolar transistor suitable for high frequency applications according to claim 1 or 2, characterized by the fact that the width of the base from the collector to the emitter is between 5 nm and 40 nm
 4. Silicon germanium hetero bipolar transistor suitable for high frequency applications according to one or more of the preceding claims, characterized by the fact that the concentration of germanium in the base layer is between 8% and 30% and preferably between 20% and 28%.
 5. Silicon germanium hetero bipolar transistor suitable for high frequency applications according to one or more of the preceding claims, characterized by the fact that the shape of the germanium concentration curve corresponds to a rectangle, a triangle or a trapezoid.
 6. Method of fabricating the epitaxial individual layers of a silicon germanium hetero bipolar transistor suitable for high frequency applications characterized in claim 1 with a silicon collector layer, a doped silicon germanium base layer and a silicon emitter layer, characterized by the fact that during fabrication of individual layers, i.e. the emitter layer (4), base layer (3) and collector layer (2) carbon is added to at least one of these layers in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ so that a point defect supported diffusion acceleration is prevented, and that the base layer is simultaneously doped with doping atoms with the relative change in the lattice constant thus introduced being less than 5-10⁻³.
 7. Method of claim 6, characterized by the fact that during one process step (A), namely vapor deposition of silicon for fabricating the collector layer, carbon is incorporated in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³
 8. Method of claim 6, characterized by the fact that during one process step (B), namely vapor deposition of silicon and additional incorporation of germanium and dopant for fabricating the base layer, carbon is incorporated in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³.
 9. Method of claim 6, characterized by the fact that during one process step (C), namely discontinuing germanium and dopant and vapor deposition of silicon for fabricating the emitter layer, carbon is incorporated in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³, the relative change in the lattice constant thus introduced being less than 5-10⁻³.
 10. Method according to one of claims 7 to 9, characterized by the fact that during process steps (A) and (B) or process steps (A) and {circle over (c)} or process steps (B) and {circle over (c)} carbon is incorporated in a concentration between 10¹⁸ cm⁻³ and 10²¹ cm⁻³
 11. Method according to one or more of the claims 7 to 10, characterized by the fact that during fabrication of the base layer (3) boron is used as the dopant in a concentration between 5-10¹⁸ cm⁻³ and 10²¹ cm⁻³.
 12. Method according to one or more of the claims 7 to 11, characterized by the fact that fabrication of the epitaxial layer is performed by a CVD process.
 13. Method according to one or more of the claims 7 to 11, characterized by the fact that fabrication of the epitaxial layer is performed by a MBE process. 